[citation needed] Primary active transport uses adenosine triphosphate (ATP) to move specific molecules and solutes against its concentration gradient.
[2] One example of passive diffusion is the gas exchange that occurs between the oxygen in the blood and the carbon dioxide present in the lungs.
[4] One example of facilitated diffusion is the movement glucose from small intestine epithelial cells into the extracellular matrix of the blood capillaries.
An example where both types of transcytosis occur is the movement of macromolecules across the blood-brain barrier (BBB) into the central nervous system (CNS).
[citation needed] In contrast, paracellular transport is the transfer of substances across an epithelium by passing through an intercellular space between the cells.